This invention relates to adhesion promotion compositions which provide excellent adhesive characteristics between rubber and textile substrates. Such compositions comprise specific mixtures of aminosilanes and other organosilanes or specific compounds having both an aminosilane moiety and a group having an affinity for rubber compositions. These inventive compositions may be utilized to activate a textile surface for further contact with a rubber alone or a resorcinol-formaldehyde latex (RFL) or similar type of adhesive compound to which a rubber may subsequently be adhered. The methods utilizing these particular adhesion promotion compositions are also contemplated within this invention.
It has long been known that adhering a textile, particularly one composed of filaments having high tenacity to a rubber enhances the modulus and tensile strength of the rubber component and provides long-lasting durability, particularly in high friction applications. Examples of such applications include fan and timing belts within automobile engines; vehicle tires; conveyor belts; and the like. The main requirement of such textile-reinforced rubber articles has been the strength of adhesion between the textile and the rubber. Without any primer treatment, the textile will not effectively adhere to the rubber. A weak bond between the two components results in separation of the two layers and mechanical failure of the resulting composite. Thus, it has been and is necessary to provide a textile treatment to enhance the adhesion of these two distinct components.
Previous methods of providing such adherence between rubber and textile layers include coating or impregnating the textile layer with a formaldehyde latex (such as a resorcinol-formaldehyde vinyl-pyridine rubber latex or RFL), pre-coating with an RFL latex and an amino functional polyacrylate, as in U.S. Pat. No. 5,654,099, to Pelton, and in European Patent Application 665,390, to Tsubakimoto Chain Company, or utilizing a pre-activated textile which has reactive pendant groups to facilitate adhesion between the fabric surface and the reactive sites on the rubber layer, all of the aforementioned references being herein entirely incorporated by reference. The RFL coating method results in a composition which does not always provide sufficient adhesion between layers. Pre-activated textiles, such as a polyester fibers coated with an epoxy adhesion enhancer, are typically used in combination with an RFL treatment to further improve the textile adhesion to rubbers. Although such pre-activated textiles perform well in many cases, there remains a need for less expensive methods and compositions for adhesion promotion between rubber and textile layers in order ultimately to produce a reinforced, long-lasting, and durable rubber product.
Also worth mentioning are U.S. Pat. No. 5,064,876, to Hamada et al., and U.S. Pat. No. 5,082,738, to Swofford, both of which teach a primer composition for promoting adhesion for polymer films. Such disclosed compositions comprise aminofunctional silanes. However, neither of these references teach the application of textiles to rubber formulations for reinforcement purposes nor do they teach or fairly suggest the necessary combination of an amine functional group and another organo-functional group, in particular a reactive group having an affinity for rubber, as is now required within the inventive composition and method.
It is thus an object of the invention to provide improved adhesion for a long-lasting and durable textile-reinforced rubber product. A further object of the invention is to provide a relatively inexpensive method of achieved such improved adhesion. Another object of the invention is to provide an adhesion promoter which ultimately provides a textile-reinforced rubber product which does not exhibit adhesive failure. Yet another object of this invention is to provide a stable aqueous primer composition that, when applied before an RFL treatment, activates the textile substrate in order to provide greatly improved adhesion to various rubber compounds.
Accordingly, this invention concerns a composition for promoting the adhesion between a rubber composition layer and a textile reinforcing layer comprising
a mixture of at least two different silane compounds, at least one compound selected from (I) and at least one compound selected from (II); wherein (I) and (II) include the following:
(I) aminosilanes selected from the group consisting of 
xe2x80x83wherein R is C1-C6 alkyl, R1 is an amine functional group, and Rxe2x80x2 is an alkyl- or aromatic-acyl group;
(II) organo-functional silanes selected from the group consisting of 
xe2x80x83wherein R2 is a group containing a radical selected from the group consisting of a mercapto moiety, a sulfide moiety, an isocyanate moiety, an epoxy moiety, a pyridine moiety, a hydrophobic saturated alkyl moiety, and a moiety having at least one unsaturated carbon-carbon bond, and R and Rxe2x80x2 are defined as for (I) above. Nowhere within the prior art has such a specific adhesion promotor composition been disclosed or practiced, particularly for activating textile substrates in order to improve adhesion to rubbers. Furthermore, nor has such a specific composition or method of utilizing such a composition been taught or fairly suggested. Such compositions and methods provide significant advantages over the standard adhesion compositions and methods of the state of the art.
Also, it has been discovered that a stable aqueous dispersion of the composition (a) above can be obtained without the use of any dispersing agents or surfactants. Component (II) of the inventive composition is not water soluble (some of the covered compounds can even react with water and thus are not stable in aqueous solution). Although a water dispersion of the component (II) can be produced utilizing surfactants, such dispersions have very limited shelf life. Besides, the surfactants necessary to make such dispersions tend to affect adhesion adversely since surfactants are known to migrate to the interfaces and interfere with the microscopic intimate contact between two uniting materials. However, by mixing components (I) and (II) together, then slowly adding water to the mixture, a stable aqueous dispersion/solution can be obtained without utilizing any potentially deleterious surfactants or dispersing agents.
The relative cost of the inventive compositions is very low at the preferred level of usage, particularly when compared with the pre-activated textiles mentioned above. All of these characteristics and improvements with the inventive compositions and methods thus translate into lower costs for the user. Additionally, the inventive compositions and methods can be utilized with any rubber compositions and with any type of smooth filament textile normally utilized as a rubber reinforcement material. Examples of rubber compositions include, and are not limited to, natural rubber, polyurethane rubber, styrene-butadiene rubber (SBR), acrylonitrile-butadiene rubber (NBR), butyl rubbers, fluorinated rubbers, and ethylene-propylene-diene rubber (EPDM). Modified rubbers which are potentially useful, though more expensive, include hydrogenated SBR, hydrogenated NBR, and carboxylated NBR. Suitable textiles include, and are not limited to, those comprising polyester, polyester/cotton blends, polyamides, such as nylon-6 or -6,6, polyaramids (such as Kevlar(copyright), available from DuPont), polypropylene, boron derivatives, glass fibers, and carbon fibers. The textile component may be dyed or colored various shades and hues in order to facilitate categorizing the different widths, lengths, etc., of products such as, without limitation, timing belts, and the like, for automobiles. Finally, the inventive compositions and methods, when utilized and/or practiced as intended, result in a textile-reinforced rubber product which does not exhibit textile/rubber adhesive failure.
Preferably, the inventive composition comprises (I) an amine-functional silane component and (II) an organo-functional silane component, which includes an unsaturated carbon-carbon bond. As noted above, other reactive groups may be employed on the organo-functional silane component, such as, isocyanates, mercaptos, and epoxies, only as examples. Being alkoxysilanes, also, both components (I) and (II) hydrolyze in the presence of water, then couple with either each other and each with itself to form oligomers. It is therefore expected that pre-hydrolyzed silane products can be utilized as substitutes for components (I) and (II). Obviously some of the organic functional group in component (II) will react with the amine group in component (I) upon mixing. However, it is the mixture of these two components that generates the unexpected synergistic adhesion improvement which cannot be achieved practically with either component alone.
Preferably, the inventive composition comprises a mixture of two separate compounds, an amine-functional silane (I) having the following structure:
R1xe2x80x94Si(OR)3xe2x80x83xe2x80x83(I)
wherein R is C1-C6 alkyl and R1 is an amine-functional group having the molecular structure of
(NH2R)(NHR)yxe2x80x94Si(OR)3
wherein R is defined for (I) as in claim 1 and y is 0-4, and
an organo-functional silane (II) having the following structure:
R2xe2x80x94Si(OR)3xe2x80x83xe2x80x83(II)
wherein R is C1-C6 alkyl and R2 is a group having an unsaturated carbon-carbon bond. Of particular preference are (I) aminoethylaminopropyltrimethoxysilane, available from Dow Corning, Midland, Mich., and (II) 3-methacryloxypropyltrimethoxysilane, also available from Dow Corning. These particular compounds provide the best adhesion promotion between textile substrates and rubber compositions for the lowest cost. Also, the amount of (I) and (II) is measured in weight ratios, with the preferred range being from about 1:1 [(I) to (II)] to about 100:1. Most preferred is a weight ratio of from about 2:1 to about 10:1. The inventive composition may be present in a dispersion, either aqueous or non-aqueous, or in a neat solution diluted with an organic solvent. An aqueous dispersion without any surfactants added is preferred, as noted previously. The preferred concentration of silanes is from about 0.1 to about 5% on the weight of the fabric (owf) with a preferred range being from about 0.5 to about 2% owf. Furthermore, it is preferred that, after the application of the inventive composition, a RFL treatment be used to increase the adhesive qualities of the entire product.
Any standard rubber additives, including ultraviolet absorbers, antioxidants, dyes, colorants, curing agents, perfumes, antistatic agents, fillers (such as carbon black), and the like may be added to the rubber. To the textile substrate may be added any other standard textile additives, such as dyes, colorants, pigments, ultra violet absorbers, antioxidants, and wetting agents. To the inventive composition and RFL used in combination with the inventive composition may be added wetting agents, antioxidants, filler dispersion (such as carbon black, silica and ZnO dispersions).
The preferred inventive method of producing a textile-reinforced rubber composite comprises the sequential steps of
(a) providing a textile substrate;
(b) contacting the textile substrate with a composition comprising either
(i) a mixture of at least two different silane compounds, one compound selected from (I) and the other selected from (II); wherein (I) and (II) include the following:
(I) amine-functional silanes selected from the group consisting essentially of 
xe2x80x83wherein R is C1-C6 alkyl, R1 is an amine-containing functional group, and Rxe2x80x2 is an alkyl- or aromatic-acyl group;
(II) organo-functional silanes selected from the group consisting essentially of 
xe2x80x83wherein R is C1-C6 alkyl, R2 is a group containing a radical selected from the group consisting essentially of a mercapto moiety, a sulfide moiety, an isocyanate moiety, an epoxy moiety, a pyridine moiety, a hydrophobic saturated alkyl moiety, and a moiety having at least one unsaturated carbon-carbon bond, and Rxe2x80x2 is the same as defined for (I) above; or
(ii) at least one functional silane containing both an amine-functional group and a moiety having an affinity for rubber (such as an unsaturated carbon-carbon bond);
(c) heating the textile substrate at a temperature of from about 120 to about 250xc2x0 C. for from about 15 seconds to about 5 minutes, preferably about 3 minutes;
(d) optionally coating or impregnating the textile substrate with an RFL composition;
(e) combining at least a portion of the resulting treated textile substrate with a rubber formulation to from a textile-reinforced rubber composite; and
(f) curing the textile-reinforced rubber composite.
In this process, contacting step (b) may be performed by any standard, well known method, including dipping, padding, spraying, coating, impregnating, and the like. This silane pre-treatment of the textile in essence xe2x80x9cactivatesxe2x80x9d the fibrous surface by effectively coating the textile with the adhesion promoter having reactive groups and groups having affinity for the rubber layer oriented in a position to facilitate the subsequent contact and adhesion of the rubber layer with the textile layer. One of ordinary skill in the art would understand and be familiar with the addition and contacting of the subject textile and rubber formulation with the potentially preferred RFL latex component as well as the rubber formulation contacting step (e), above. Furthermore, curing step (f) is performed in any conventional manner, such as through heat-activated vulcanization in the presence of a curing agent (such as organic peroxide). Again, such a step should be well within the purview of the ordinarily skilled artisan in this field.